This invention relates to a carburetor of an internal combustion engine, and more particularly relates to a variable venturi type carburetor having a fuel injection valve in which, during low-load operation with a low amount of suction fuel, the atomization or mixing characteristic is improved and the change in the amount of fuel being fed into the engine combustion chambers is reduced.
A variable venturi type carburetor having a fuel injection valve comprises a suction piston movably mounted on a venturi portion of an intake passage, so as to vary the cross-sectional area of the venturi in accordance with the change in the amount of air being fed to the engine combustion chambers. As is well-known, the cross-sectional area of the venturi portion is controlled so that the velocity of air flowing in the venturi, that is, the vacuum level in the venturi, is always maintained at a constant value. Fuel is injected into the venturi portion from the fuel injection valve provided in the vicinity of the venturi portion, and atomized so as to be mixed with sucked air. The fuel injection valve is usually controlled electrically in accordance with the running conditions of the engine, such as, the amount of sucked air, the rate of engine revolution speed, the temperature of sucked air, the temperature of engine cooling water and so on; and/or in accordance with the running conditions of the engine measured by the signals form an O.sub.2 sensor provided in an exhaust passage, so that the necessary amount of fuel is supplied wherein the actual air-fuel ratio of the mixture is in the vicinity of the so called theoretical air fuel-ratio.
In a variable venturi type carburetor having a fuel injection valve, the speed of the fuel injected from the fuel injection valve is constant, since the speed of the sucked air flowing through the venturi portion is always constant, and the pressure of injected fuel and the absolute pressure in the venturi portion are also constant. Therefore, the streamline of the injected fuel draws a certain curve along which the fuel is sucked downwardly in the intake passage. In addition, as the fuel injection valve is actuated and electrically controlled with the signal pulse, the fuel injection takes place periodically.
In a conventional variable venturi type carburetor, as the surface of the head portion of the suction piston which defines the cross-sectional area of the venturi portion is flat, during low-load operation with a low amount of sucked air wherein the cross-sectional area of the venturi portion is small, the distance from an injection nozzle of the fuel injection valve to the head portion of the suction piston is relatively small, so that the injected fuel may be attached on the head portion of the suction piston and fall in drops downwardly in the intake passage. As a result, a rich mixture and lean mixture are alternately supplied into the combustion chambers of the engine, so that smooth operation of the engine is disturbed. In addition, during the low load operation of the engine, the cross-sectional area of the venturi portion has a shape of an elongated rectangular or semi-circular slit, so that the rate of mixing of air and fuel at the central part of the intake passage is different from the rate at the peripheral part of the passage. Consequently, a homogeneous mixture of air and fuel cannot be obtained, the fuel combustion in the engine cannot be stabilized and misfires may occur, so that smooth operation of the engine is disturbed. Further, the rate of fuel consumption during the low load operation increases, so that a large quantity of noxious substances, such as hydrocarbons (HC) and carbon monoxide (CO) are contained, in the exhaust gas.